[PATCH v2] iio: Fix scan mask subset check logic

[Fabio Estevam <festevam@gmail.com>]

From: Fabio Estevam <festevam@denx.de>

Since commit 2718f15403fb ("iio: sanity check available_scan_masks array"),
verbose and misleading warnings are printed for devices like the MAX11601:

max1363 1-0064: available_scan_mask 8 subset of 0. Never used
max1363 1-0064: available_scan_mask 9 subset of 0. Never used
max1363 1-0064: available_scan_mask 10 subset of 0. Never used
max1363 1-0064: available_scan_mask 11 subset of 0. Never used
max1363 1-0064: available_scan_mask 12 subset of 0. Never used
max1363 1-0064: available_scan_mask 13 subset of 0. Never used
...
[warnings continue] 

Adding some debug prints show that the mask1 and mask2 indexes
are incorrectly computed, leading to wrong array access.

These incorrect accesses causes the wrong subset output:

iio-core: ******** mask1[0] is 0x1
iio-core: ******** mask2[1] is 0x4
iio-core: ******** mask2[2] is 0x3
iio-core: ******** mask2[3] is 0xf
iio-core: ******** mask2[4] is 0x1000
iio-core: ******** mask2[5] is 0x40000
iio-core: ******** mask2[6] is 0x3000
iio-core: ******** mask2[7] is 0x0
iio-core: ******** mask2[8] is 0xffff0000000e2720
max1363 1-0064: available_scan_mask 8 subset of 0. Never used
iio-core: ******** mask2[9] is 0xffff0000000e2660
max1363 1-0064: available_scan_mask 9 subset of 0. Never used
iio-core: ******** mask2[10] is 0xffff0000000e25a0
max1363 1-0064: available_scan_mask 10 subset of 0. Never used
iio-core: ******** mask2[11] is 0xffff0000000e24e0
max1363 1-0064: available_scan_mask 11 subset of 0. Never used
iio-core: ******** mask2[12] is 0xffff0000000e2420
max1363 1-0064: available_scan_mask 12 subset of 0. Never used
iio-core: ******** mask2[13] is 0xffff0000000e2360
max1363 1-0064: available_scan_mask 13 subset of 0. Never used

Fix the available_scan_masks sanity check logic so that it
only prints the warning when an element of available_scan_mask
is in fact a subset of a previous one.

With this fix, the warning output becomes both correct and more
informative:

max1363 1-0064: Mask 7 (0xc) is a subset of mask 6 (0xf) and will be ignored.

Cc: stable@vger.kernel.org
Fixes: 2718f15403fb ("iio: sanity check available_scan_masks array")
Signed-off-by: Fabio Estevam <festevam@denx.de>
---
Changes since v1:
- Make it a standalone patch. max1363 fix will be sent later.
- Keep using bitmap_subset().
- Add more information about the error in the commit log.

 drivers/iio/industrialio-core.c | 18 +++++-------------
 1 file changed, 5 insertions(+), 13 deletions(-)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index 6a6568d4a2cb..dabfcb50e5fe 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -1947,21 +1947,13 @@ static void iio_sanity_check_avail_scan_masks(struct iio_dev *indio_dev)
 	 * available masks in the order of preference (presumably the least
 	 * costy to access masks first).
 	 */
-	for (i = 0; i < num_masks - 1; i++) {
-		const unsigned long *mask1;
-		int j;
 
-		mask1 = av_masks + i * longs_per_mask;
-		for (j = i + 1; j < num_masks; j++) {
-			const unsigned long *mask2;
-
-			mask2 = av_masks + j * longs_per_mask;
-			if (bitmap_subset(mask2, mask1, masklength))
+	for (i = 1; i < num_masks; i++)
+		for (int j = 0; j < i; j++)
+			if (bitmap_subset(&av_masks[i], &av_masks[j], BITS_PER_LONG))
 				dev_warn(indio_dev->dev.parent,
-					 "available_scan_mask %d subset of %d. Never used\n",
-					 j, i);
-		}
-	}
+					 "Mask %d (0x%lx) is a subset of mask %d (0x%lx) and will be ignored.\n",
+					 i, av_masks[i], j, av_masks[j]);
 }
 
 /**
-- 
2.34.1